1. Field of the Invention
This invention relates in general to wireless digital communications. In particular, the invention relates to a data frame structure for use in a wireless communications system, such as a single or multiple-handset cordless telephone system.
2. Background Art
Wireless telephone devices have become increasingly popular among individuals, finding use in many applications across both commercial and private sectors. The designers of modern telephone systems have embraced the use of digital technology to provide additional features, improved performance and increased reliability for the subscribers of the various systems. Whether it is a single-handset cordless phone used in the home, an enterprise-wide multiple-handset cordless phone system for a large corporation or one of the ubiquitous cellular phones, the vast majority of these systems have transitioned to, or are in the process of transitioning to, one of the numerous recognized digital communication standards.
Digital telephone manufacturers have a wide variety of digital technologies from which to choose when designing digital phone systems with each technology offering its own advantages. One such digital communication standard is Time Division Multiple Access, or TDMA. TDMA allows multiple users to communicate on the same radio frequency by transmitting bursts of encoded data at distinct, pre-determined moments in time, referred to as timeslots. TDMA technology is frequently used in implementing cellular and both single- and multiple-handset cordless telephone systems, as well as other communication systems. A related technology is Time Division-Duplex (TDD). TDD systems carry both transmit and receive data on the same frequency channel, with the two communicating units taking turns alternately transmitting and receiving bursts of encoded data at successive moments in time. This is shown graphically in the single-channel cordless telephone TDD frame structure of FIG. 1. A single-handset cordless phone system is illustrated wherein the base station (BS) first transmits to the handset (HS) 100, which is then followed by the handset reply 101. The Received Signal Strength Indicator (RSSI) period 102 at the end of the frame is used to measure the level of interference on any particular frequency for interference mitigation, and is optional. The shaded areas indicate guard bands 103a, 103b and 103c to allow for frequency and switching settling during which no data transmission occurs. Communication systems that use TDMA and TDD technologies benefit from improved performance as compared to the performance of older analog communication systems.
Designers continually work to improve the quality and capacity of digital communication systems, including TDMA and TDD systems. One way in which system performance can be improved is through the use of frequency hopping. A frequency hopping radio system is one that transmits data (which in the context of cordless phones includes voice traffic) over a sequence of different carrier frequencies. At any one time, only one frequency is used but this frequency changes (hops) in the time domain. The sequence of frequencies used is known as the hop pattern.
Interference is always a concern in any communication system, and a frequency hopping communication system is no exception. Interference might take the form of a non-time-varying interfering signal, such as a fixed-frequency transmitter operating within the same frequency range as the hopping system, or a time-varying interference signal, such as another hopping system operating within the same band as the first hopping system.
One way in which the effects of fixed-frequency or slowly time-varying interference can be mitigated is through the use of frequency adaptation techniques. Once a system senses the presence of a steady interfering signal, the hopping frequencies that coincide with the interfering signal can be avoided. However, interference that varies in time at a rate similar to or faster than the hop speed of the link in question typically cannot be avoided by such frequency adaptation techniques because the frequency of the interfering signal cannot be predicted.
Another possible technique to combat interference and provide for more robust signal reception is the use of spatial diversity. Spatial diversity is created within a communications system when multiple physical paths are used to transmit the same information to its destination. This can be accomplished by using two separate antennas connected to two individual receivers that process the received signal. Because the signals inevitably take different paths to arrive at the physically separate receive antennas, the signals will be attenuated to different degrees by interference, fading or other phenomenon. The system can then select the stronger of the two received signals or combine the two signals in some fashion to provide the best possible received signal.
However, the implementation of such spatial diversity systems often increases the cost, increases physical size and power consumption requirements, and may not be appropriate for consumer products such as cordless telephones. More importantly, typical spatial diversity systems may not adequately address the interference challenges presented by other frequency hopping systems operating within the same frequency range.
Other common interference avoidance techniques rely upon the careful selection of filters such as ceramic, SAW, and cavity filters which are effective against known sources of interference that exist outside the operating bandwidth of the communication system, but typically cannot address interference signals operating in-band. Furthermore, complex interference cancellation algorithms have been employed in some systems to address in-band interference, but the efficacy of these techniques is often doubtful while the processing power required to implement them may be significant, with high development costs, making such algorithms undesirable for many consumer communication systems.
Thus, there exists a need to provide a low-cost, easy-to-implement solution that is effective against time varying interference for consumer communication systems such as cordless telephone systems and other systems that use TDMA TDD technology.